This invention relates to a device for measuring the rotational speed of a diesel engine.
The rotational speed of a gasoline engine, which uses an electric ignition, can easily be measured by utilizing an electric signal from the igniting source. However, a diesel engine does not use an electric ignition and, therefore an electric signal from the igniting source can not be utilized in a diesel engine. To overcome such inconvenience, a device for measuring the rotational speed of mechanical type or electromechanical type (dynamo type) is used and pressed against a rotational shaft of a diesel engine so that the rotational speed of the diesel engine is measured. In another method, a reflecting tape or magnetic substance is secured to the rotational portion of the diesel engine so that the rotational speed of the diesel engine is measured by means of a photoelectric type rotational meter or an electromagnetic type rotational meter which is disposed at a position adjacent to the rotational portion.
However, since a diesel engine which is installed in a vehicle does not have a rotational portion which is exposed to the outside, it is very difficult to easily press a device against the rotational shaft of the diesel engine or to readily secure a reflecting tape or magnetic substance to the rotational shaft of the diesl engine. As a result, the above-mentioned measuring methods are not desirable for practical use. Since in many automobile repair shops the rotational speed of a diesel engine cannot be measured easily in a short time, the rotational speed of a diesel engine is often estimated by listening to the sound emitted from the diesel engine. In a case where a reflecting tape or magnetic substance can be secured to a rotational shaft of a diesel engine, since each of the above-mentioned devices for measuring rotational speed requires one operator who must always check the device during the measuring operation, an operation wherein a function of a part is checked at a predetermined rotational speed, for example, the checking of a regulator installed in a charge dynamo at a predetermined constant rotational speed, must be effected by at least two operators and this results poor labor efficiency. Furthermore, oil or dust may adhere to the surface of the reflecting tape or magnetic substance, and then the measuring characteristics of the device is lowered. Therefore, the above-mentioned devices are not suitable for measuring the rotational speed of a diesel engine over a long period of time by installing the devices on a vehicle in which the engine is installed.
Recently, a method has been proposed for measuring the rotational speed of a diesel engine, in which method pulse-like mechanical pressure changes, each of which is generated when fuel is injected, are detected by a pressure sensing element which is installed within a fuel injection nozzle for supplying fuel into fuel chambers of the diesel engine or within a delivery pipe connecting a fuel injection nozzle with an injection pump. However, according to this method, since it is necessary to locate the pressure sensing element within the fuel injection nozzle or delivery pipe, the fuel nozzle or delivery pipe must be dismounted from the diesel engine and mounted again on the engine whenever the rotational speed of the diesel engine is measured. Therefore, this method is very troublesome. In addition, since the injection nozzle and delivery pipe influence the operating characteristics of the diesel engine, the operating characteristics of the diesel engine are degraded when the injection nozzle and delivery pipe is not adequately mounted and adjusted. Consequently, this method requires a special skill for mounting the sensing element and is not desirable for practical use. Furthermore, since the sizes of the injection nozzles and delivery nozzles are different from each other in accordance with the types of vehicles, various types of the injection nozzles and delivery pipes, both of which have sensing elements mounted therein, must be prepared when the rotational speed of various types of diesel engines is measured in a repair shop. This is not economical.
Another method for measuring the rotational speed of a diesel engine is proposed in Japanese Utility Model Application Laid Open No. 277-77. In this method, local pressure changes generated in a delivery pipe when fuel is injected under a high injecting pressure are detected as electric signals by means of an oscillator which has an intrinsic reasonance frequency and which is disposed in abutment with the delivery pipe, so that the rotational speed of the engine is measured by utilizing the electric signal. According to this method, dismounting and mounting of the fuel injection nozzle and delivery pipe, and the adjustment required in the mounting and dismounting become unnecessary. As a result, it would normally be expected that the rotational speed of a diesel engine can be easily measured without causing a degradation of the operating characteristics of the diesel engine. However, the device disclosed in Japanese Utility Model Application Laid Open No. 277-77 has some defects in the specifications thereof, especially the allowable range of intrinsic reasonance frequency and construction, and in the processing of the electric signals obtained. Therefore, the device is not preferable for the practical use.